#23 multiple series on same view

[Serge Rey]

for densities, box plots and possibly other views, it
might be useful to have an option to either:

generate multiple instances of a view, with each
instance a new time period or spatial unit

or

have a single instance with multiple series represented
on that instance.

an example of the second would be a density view that
has kernels for each decade drawn over the same x-y axes.